Identification of Disease Causing Variants in Genes Responsible for Hereditary Skeletal Disorders

Abstract

Hereditary skeletal dysplasias include a diverse group of genetically inherited disorders usually characterized by structural and functional abnormality of bones and carti lages growth and modeling. Bones development and modeling involves various growth and morphogenetic factors that regulate osteoblast and osteoclast activity. Skeletal hereditary disorders are caused by mutations in genes encoding these proteins with a role in growth and function of bone. To this date, mutations in 364 genes have been identified to be involved in a total of 436 so far diagnosed skeletal disorders. The inheritance pattern of these disorders can be autosomal dominant, autosomal recessive or X-linked . The frequency of skeletal disorders increases with successive intermarriages in a generation. In Pakistani population consanguineous marriages are common due to social, ethnic and traditional customs. In the present study, three families (A, B, C) showing clinically distinct hereditary skeletal disorders were evaluated genetically. Affected individuals in family A showed autosomal recessive Acromesomelic dysplasia. In Family B, Affected members showed Autosomal recessive mesoaxial synostotic syndactyly while Family C showed phenotypes of Brachydactyly type B 1. To track the mutated genes responsible for specific disorder, homozygosity mapping based on Sequence-Tagged Site (STS) markers was carried out in above mentioned families. The linked gene was then sequenced by Sanger sequencing in both affected and normal members of the respective families. Linkage in family A, with autosomal recessIve acromesomelic dysplasia, was establ ished to NP R2 gene on chromosome 9p 13.3. Sanger sequencing showed a novel nonsense mutation in exon 22 distinguished by substitution of Cytosine by Thiamine at c.DNA position 3094 (c.3094C>T). Mutation of this sequence leads to formation of a stop codon at amino acid position 1 032 (p.Argl032*) resulting in a truncated and non-functional NPR-B protein. In family B All the known genes for various types of mesoaxial synostotic syndactyly were checked and none of them was linked, which signifies the involvement of a novel gene in causing this particular disorder. In family C, ROR2 gene on clu'omosome 9q22.31 , was directly sequenced. Sequence analysis of seven (2, 3, 4, 5, 7, 8, 9) out of nine exons of ROR2 revealed a heterozygous variant c. 1712A>G (p.His57 1 Arg) in exon 9 of the gene with minor allele frequency of O.99999.The remaining two exons (exon 1,6) sequencing is under process, possibly the disease-causing vari ant lay there.